My bottom grate sits on top of the supports for the water bowl. IOW, as low as I can get it and still have room for a bowl or whatever you plan to use between coals and bottom grate. (I have cut the bottom out rather than drill holes.)
My top grate is just below the top edge of the cooker and is too high. I lack clearance under the lid. Some day when I'm a little more energetic I'm going to measure the distance between top grate and lid and between the grates, split the difference and remount the supports for the top grate.